Polymer electrolyte membranes crosslinked by nitrile trimerization

ABSTRACT

A method is provided for making a crosslinked polymer electrolyte, typically in the form of a membrane for use as a polymer electrolyte membrane in an electrolytic cell such as a fuel cell, by trimerization of nitrile groups contained on groups pendant from the polymer. The resulting polymer electrolyte membrane comprises a highly fluorinated polymer comprising: a perfluorinated backbone, first pendent groups which comprise sulfonic acid groups, and crosslinks comprising trivalent groups according to the formula:  
                 
 
The first pendent groups are typically according to the formula: —R 1 —SO 3 H, where R 1  is a branched or unbranched perfluoroalkyl or perfluoroether group comprising 1-15 carbon atoms and 0-4 oxygen atoms, most typically —O—CF 2 —CF 2 —CF 2 —CF 2 —SO 3 H or —O—CF 2 —CF(CF 3 )—O—CF 2 —CF 2 —SO 3 H.

FIELD OF THE INVENTION

This invention relates to a method of making a crosslinked polymerelectrolyte, typically in the form of a membrane for use as a polymerelectrolyte membrane in an electrolytic cell such as a fuel cell, bytrimerization of nitrile groups contained on groups pendant from thepolymer.

BACKGROUND OF THE INVENTION

International Patent Application Publication No. WO 02/50142 A1purportedly discloses fluorosulphonated nitrile crosslinkable elastomersbased on vinylidene fluoride with low Tg.

U.S. Pat. No. 5,260,351 purportedly discloses perfluoroelastomers curedby radiation in the absence of curing agents. The reference purportedlyrelates to curing of fully fluorinated polymers, such as those preparedfrom tetrafluoroethylene, a perfluoralkyl perfluorovinyl ether, and curesite or crosslinking units providing at least one of nitrile,perfluorophenyl, bromine or iodine in the resulting terpolymer.

U.S. Pat. No. 5,527,861 purportedly discloses nitrile containingperfluoroelastomers cured by a combination of a peroxide, a coagent, anda catalyst which causes crosslinks to form using the nitrile groups.

U.S. Pat. Nos. 4,334,082, 4,414,159, 4,440,917, and 4,454,247purportedly disclose an ion exchange membrane for use in a chlor-alkalielectrolysis cell formed from a copolymer of a vinyl ether monomer ofthe formula:Y₂CFO(CF(CF₃)CF₂O)_(n)CF═CF₂where Y is selected from the group consisting of CF₂CN, CF₂CO₂R,CF₂CO₂H, CF₂CO₂M, CF₂CONH₂ and CF₂CONR; a perfluorinated comonomerselected from tetrafluoroethylene, hexafluoropropylene, andperfluoroalkylvinyl ether; andCF₂═CF(OCF₂CF(CF₃))_(n)OCF₂CF₂SO₂Fwhere n is 1 or 2. (U.S. Pat. No. 4,454,247 at claim 1). Thesereferences purportedly disclose a method of curing fluoroelastomers bytrimerization of nitrites to form triazine rings. (U.S. Pat. No.4,454,247 at col. 10, lns. 60-68).

SUMMARY OF THE INVENTION

Briefly, the present invention provides a polymer electrolyte membranecomprising a highly fluorinated polymer comprising: a perfluorinatedbackbone, first pendent groups which comprise sulfonic acid groups, andcrosslinks comprising trivalent groups according to the formula:

The first pendent groups are typically according to the formula:—R¹—SO₃H, where R¹ is a branched or unbranched perfluoroalkyl orperfluoroether group comprising 1-15 carbon atoms and 0-4 oxygen atoms,most typically —O—CF₂—CF₂—CF₂—CF₂—SO₃H or —O—CF₂—CF(CF₃)—O—CF₂—CF₂—SO₃H.

In another aspect, the present invention provides a method of making apolymer electrolyte membrane comprising the steps of: a) providing ahighly fluorinated polymer comprising: a perfluorinated backbone, firstpendent groups which comprise sulfonyl halide groups, and second pendentgroups which comprise nitrile groups; b) forming the fluoropolymer intoa membrane; c) forming crosslinks by trimerization of the nitrilegroups; and d) converting the sulfonyl halide groups to sulfonic acidgroups. The second pendent groups are typically according to theformula: —C≡N or —R¹¹—C≡N, where R¹¹ is a branched or unbranchedperfluoroalkyl or perfluoroether group comprising 1-15 carbon atoms and0-4 oxygen atoms. The first pendent groups typically according to theformula: —R¹—SO₂X, where X is a halogen and where R¹ is a branched orunbranched perfluoroalkyl or perfluoroether group comprising 1-15 carbonatoms and 0-4 oxygen atoms, most typically —O—CF₂—CF₂—CF₂—CF₂—SO₂X or—O—CF₂—CF(CF₃)—O—CF₂—CF₂—SO₂X.

In another aspect, the present invention provides crosslinked polymersmade according to any of the methods of the present invention.

In this application:

-   -   “equivalent weight” (EW) of a polymer means the weight of        polymer which will neutralize one equivalent of base;    -   “hydration product” (HP) of a polymer means the number of        equivalents (moles) of water absorbed by a membrane per        equivalent of sulfonic acid groups present in the membrane        multiplied by the equivalent weight of the polymer; and    -   “highly fluorinated” means containing fluorine in an amount of        40 wt % or more, typically 50 wt % or more and more typically 60        wt % or more.

DETAILED DESCRIPTION

The present invention provides a crosslinked polymer electrolytemembrane. The membrane is derived from a polymer comprising: a highlyfluorinated backbone, first pendent groups which include a groupaccording to the formula —SO₂X, where X is a halogen and second pendentgroups which include a nitrile group (—C≡N) which may be trimerized toform crosslinks comprising trivalent groups according to the formula:

Such crosslinked polymer electrolyte membranes (PEM's) may be used inelectrolytic cells such as fuel cells.

PEM's manufactured from the crosslinked polymer according to the presentinvention may be used in the fabrication of membrane electrodeassemblies (MEA's) for use in fuel cells. An MEA is the central elementof a proton exchange membrane fuel cell, such as a hydrogen fuel cell.Fuel cells are electrochemical cells which produce usable electricity bythe catalyzed combination of a fuel such as hydrogen and an oxidant suchas oxygen. Typical MEA's comprise a polymer electrolyte membrane (PEM)(also known as an ion conductive membrane (ICM)), which functions as asolid electrolyte. One face of the PEM is in contact with an anodeelectrode layer and the opposite face is in contact with a cathodeelectrode layer. Each electrode layer includes electrochemicalcatalysts, typically including platinum metal. Gas diffusion layers(GDL's) facilitate gas transport to and from the anode and cathodeelectrode materials and conduct electrical current. The GDL may also becalled a fluid transport layer (FTL) or a diffuser/current collector(DCC). The anode and cathode electrode layers may be applied to GDL's inthe form of a catalyst ink, and the resulting coated GDL's sandwichedwith a PEM to form a five-layer MEA. Alternately, the anode and cathodeelectrode layers may be applied to opposite sides of the PEM in the formof a catalyst ink, and the resulting catalyst-coated membrane (CCM)sandwiched with two GDL's to form a five-layer MEA. The five layers of afive-layer MEA are, in order: anode GDL, anode electrode layer, PEM,cathode electrode layer, and cathode GDL. In a typical PEM fuel cell,protons are formed at the anode via hydrogen oxidation and transportedacross the PEM to the cathode to react with oxygen, causing electricalcurrent to flow in an external circuit connecting the electrodes. ThePEM forms a durable, non-porous, electrically non-conductive mechanicalbarrier between the reactant gases, yet it also passes H⁺ ions readily.

The polymer to be crosslinked comprises a backbone, which may bebranched or unbranched but is typically unbranched. The backbone ishighly fluorinated and more typically perfluorinated. The backbone maycomprise units derived from tetrafluoroethylene (TFE), i.e., typically—CF₂—CF₂— units, and units derived from co-monomers, typically includingat least one according to the formula CF₂═CY—R where Y is typically Fbut may also be CF₃, and where R is a first pendant group which includesa group according to the formula —SO₂X which is a sulfonyl halide. X ismost typically F. In an alternative embodiment, first side groups R maybe added to the backbone by grafting. Typically, first side groups R arehighly fluorinated and more typically perfluorinated. R may be aromaticor non-aromatic. Typically, R is —R¹—SO₂X, where R¹ is a branched orunbranched perfluoroalkyl or perfluoroether group comprising 1-15 carbonatoms and 0-4 oxygen atoms. R¹ is typically —O—R²— wherein R² is abranched or unbranched perfluoroalkyl or perfluoroether group comprising1-15 carbon atoms and 0-4 oxygen atoms. R¹ is more typically —O—R³—wherein R³ is a perfluoroalkyl group comprising 1-15 carbon atoms.Examples of R¹ include:

-   -   —(CF₂)_(n)— where n is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,        13, 14 or 15    -   (—CF₂CF(CF₃)—)_(n) where n is 1, 2, 3, 4, or 5    -   (—CF(CF₃)CF₂—)_(n) where n is 1, 2, 3, 4, or        5(—CF₂CF(CF₃)—)_(n)—CF₂— where n is 1, 2, 3 or 4    -   (—O—CF₂CF₂—)_(n) where n is 1, 2, 3, 4, 5, 6 or 7    -   (—O—CF₂CF₂CF₂—)_(n) where n is 1, 2, 3, 4, or 5    -   (—O—CF₂CF₂CF₂CF₂—)_(n) where n is 1, 2 or 3    -   (—O—CF₂CF(CF₃)—)_(n) where n is 1, 2, 3, 4, or 5    -   (—O—CF₂CF(CF₂CF₃)—)_(n) where n is 1, 2 or 3    -   (—O—CF(CF₃)CF₂—)_(n) where n is 1, 2, 3, 4 or 5    -   (—O—CF(CF₂CF₃)CF₂—)_(n) where n is 1, 2 or 3    -   (—O—CF₂CF(CF₃)—)_(n)—O—CF₂CF₂— where n is 1, 2, 3 or 4    -   (—O—CF₂CF(CF₂CF₃)—)_(n)—O—CF₂CF₂— where n is 1, 2 or 3    -   (—O—CF(CF₃)CF₂—)_(n)—O—CF₂CF₂— where n is 1, 2, 3 or 4    -   (—O—CF(CF₂CF₃)CF₂—)_(n)—O—CF₂CF₂— where n is 1, 2 or 3    -   —O—(CF₂)_(n)— where n is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,        13 or 14    -   R is typically —O—CF₂CF₂CF₂CF₂—SO₂X or        —O—CF₂—CF(CF₃)—O—CF₂—CF₂—SO₂X and most typically        —O—CF₂CF₂CF₂CF₂—SO₂X. The —SO₂X group is most typically —SO₂F        during polymerization, i.e., X is F. The —SO₂X group is        typically converted to —SO₃H at some point prior to use of the        fluoropolymer as an polymer electrolyte.

The fluoromonomer providing first side group R may be synthesized by anysuitable means, including methods disclosed in U.S. Pat. No. 6,624,328.

In addition, the fluoropolymer includes second pendant groups Qcontaining a —C≡N group. The second pendant group may be derived from aco-monomer according to the formula CF₂═CY-Q where Y is typically F butmay also be CF₃, and where Q is a second pendent group which includes agroup according to the formula —C≡N. In an alternative embodiment,second pendant groups Q may be added to the backbone by grafting.Typically, second pendant groups Q are highly fluorinated and moretypically perfluorinated, other than at the bromine position. Typically,Q is —R¹¹—C≡N, where R¹¹ can be any R¹ described above but is selectedindependently from R¹.

Most typically, the fluoropolymer is a terpolymer of TFE, CF₂═CY—R asdescribed above, and CF₂═CY-Q as described above.

The polymer to be crosslinked may be made by any suitable method,including emulsion polymerization, extrusion polymerization,polymerization in supercritical carbon dioxide, solution or suspensionpolymerization, and the like. In one typical polymerization,CF₂═CF—O—CF₂CF₂CF₂CF₂—SO₂F (MV4S) is preemulsified in water with anemulsifier (ammonium perfluorooctanoate, C₇F₁₅COONH₄) under high shear(24,000 rpm). An oxygen-free polymerization kettle equipped with animpeller agitator system is charged with deionized water and heated to50° C. and then the preemulsion is charged into the polymerizationkettle. The kettle is charged with a nitrile-functional monomer such asCF₂═CF—O—C₅F₁₀—C≡N, typically in a preemulsion. The kettle is furthercharged with gaseous tetrafluoroethylene (TFE) to 6-8 bar absolutereaction pressure. At 50° C. and 240 rpm agitator speed polymerizationis initiated by addition of sodium disulfite and ammoniumperoxodisulfate. During the course of the reaction, the reactiontemperature is maintained at 50° C. Reaction pressure is maintained at6-8 bar absolute by feeding additional TFE into the gas phase. A secondportion of MV4S preemulsion may be continuously fed into the liquidphase during the course of the reaction. Additional nitrile-functionalmonomer may also be continuously fed into the reactor during the courseof the reaction. After feeding sufficient TFE, the monomer feed may beinterrupted and the continuing polymerization allowed to reduce thepressure of the monomer gas phase. The reactor may then be vented andflushed with nitrogen gas.

Typically, the polymer is formed into a membrane prior to crosslinking.Any suitable method of forming the membrane may be used. The polymer istypically cast from a suspension. Any suitable casting method may beused, including bar coating, spray coating, slit coating, brush coating,and the like. Alternately, the membrane may be formed from neat polymerin a melt process such as extrusion. After forming, the membrane may beannealed. Typically the membrane has a thickness of 90 microns or less,more typically 60 microns or less, and most typically 30 microns orless. A thinner membrane may provide less resistance to the passage ofions. In fuel cell use, this results in cooler operation and greateroutput of usable energy. Thinner membranes must be made of materialsthat maintain their structural integrity in use. In one typical process,membranes are cast by knife coating out of a water suspension containing20% solids onto a glass plate and dried at 80° C. for 10 minutes to formfilms having a thickness of approximately 30 microns.

The step of crosslinking (nitrile trimerization) may be accomplished byany suitable method. Typically, crosslinking is accomplished byapplication of heat, typically to a temperature of 160° C. or more, inthe presence of suitable initiators or catalysts. Suitable initiators orcatalysts may include ammonia, ammonium compounds, including salts ofammonium and salts of quaternary ammonium compounds, including cycliccompounds such as salts of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU),including salts of fluorinated carboxylates, Lewis acids, and the like.The step of crosslinking the polymer may occur in whole or part duringannealing of the membrane or may be carried out separate from anyannealing step. During the crosslinking step, nitrile groups trimerizeto form linkages comprising triazine groups, i.e., trivalent groupsaccording to the formula:

After crosslinking, the sulfur-containing functions of the first pendantgroups may be converted to sulfonic acid form by any suitable process.Sulfonyl halide groups may be converted by hydrolysis. In one typicalprocess, the polymer is immersed in an aqueous solution of a strong baseand subsequently acidified. In one typical embodiment, a polymermembrane is immersed in 15% KOH in water at 80° C. for 1 hour, thenwashed twice in 20% nitric acid at 80° C., then boiled in deionizedwater twice.

The acid-functional pendent groups typically are present in an amountsufficient to result in an hydration product (HP) of greater than15,000, more typically greater than 18,000, more typically greater than22,000, and most typically greater than 25,000. In general, higher HPcorrelates with higher ionic conductance.

The acid-functional pendent groups typically are present in an amountsufficient to result in an equivalent weight (EW) of less than 1200,more typically less than 1100, and more typically less than 1000, andmore typically less than 900.

In a further embodiment, the polymer may be imbibed into a poroussupporting matrix prior to crosslinking, typically in the form of a thinmembrane having a thickness of 90 microns or less, more typically 60microns or less, and most typically 30 microns or less. Any suitablemethod of imbibing the polymer into the pores of the supporting matrixmay be used, including overpressure, vacuum, wicking, immersion, and thelike. The blend becomes embedded in the matrix upon crosslinking. Anysuitable supporting matrix may be used. Typically the supporting matrixis electrically non-conductive. Typically, the supporting matrix iscomposed of a fluoropolymer, which is more typically perfluorinated.Typical matrices include porous polytetrafluoroethylene (PTFE), such asbiaxially stretched PTFE webs.

It will be understood that polymers and membranes made according to themethod of the present invention may differ in chemical structure fromthose made by other methods, in the structure of crosslinks, theplacement of crosslinks, the placement of acid-functional groups, thepresence or absence of crosslinks on pendent groups or ofacid-functional groups on crosslinks, and the like.

This invention is useful in the manufacture of strengthened polymerelectrolyte membranes for use in electrolytic cells such as fuel cells.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand principles of this invention, and it should be understood that thisinvention is not to be unduly limited to the illustrative embodimentsset forth hereinabove.

1. A polymer electrolyte membrane comprising a highly fluorinatedpolymer comprising: a perfluorinated backbone, first pendent groupswhich comprise sulfonic acid groups, and crosslinks comprising trivalentgroups according to the formula:


2. The polymer electrolyte membrane according to claim 1 wherein saidfirst pendent groups are according to the formula: —R¹—SO₃H, where R¹ isa branched or unbranched perfluoroalkyl or perfluoroether groupcomprising 1-15 carbon atoms and 0-4 oxygen atoms.
 3. The polymerelectrolyte membrane according to claim 1 wherein said first pendentgroups are according to the formula: —O—CF₂—CF₂—CF₂—CF₂—SO₃H.
 4. Thepolymer electrolyte membrane according to claim 1 wherein said firstpendent groups are according to the formula:—O—CF₂—CF(CF₃)—O—CF₂—CF₂—SO₃H.
 5. A method of making a polymerelectrolyte membrane comprising the steps of: a) providing a highlyfluorinated polymer comprising: a perfluorinated backbone, first pendentgroups which comprise sulfonyl halide groups, and second pendent groupswhich comprise nitrile groups; b) forming said fluoropolymer into amembrane; c) trimerizing said nitrile groups to form crosslinks; and d)converting said sulfonyl halide groups to sulfonic acid groups.
 6. Themethod according to claim 5 wherein said second pendent groups areselected from —C≡N and groups according to the formula: —R¹—C≡N, whereR¹ is a branched or unbranched perfluoroalkyl or perfluoroether groupcomprising 1-15 carbon atoms and 0-4 oxygen atoms.
 7. The methodaccording to claim 5 wherein said first pendent groups are according tothe formula: —R¹—SO₂X, where X is a halogen and where R¹ is a branchedor unbranched perfluoroalkyl or perfluoroether group comprising 1-15carbon atoms and 0-4 oxygen atoms.
 8. The method according to claim 6wherein said first pendent groups are according to the formula:—R¹—SO₂X, where X is a halogen and where R¹ is a branched or unbranchedperfluoroalkyl or perfluoroether group comprising 1-15 carbon atoms and0-4 oxygen atoms.
 9. The method according to claim 7 wherein said firstpendent groups are according to the formula: —O—CF₂—CF₂—CF₂—CF₂—SO₂X.10. The method according to claim 8 wherein said first pendent groupsare according to the formula: —O—CF₂—CF₂—CF₂—CF₂—SO₂X.
 11. The methodaccording to claim 7 wherein said first pendent groups are according tothe formula: —O—CF₂—CF(CF₃)—O—CF₂—CF₂—SO₂X.
 12. The method according toclaim 8 wherein said first pendent groups are according to the formula:—O—CF₂—CF(CF₃)—O—CF₂—CF₂—SO₂X.
 13. A polymer electrolyte membrane madeaccording to the method of claim
 5. 14. A polymer electrolyte membranemade according to the method of claim
 6. 15. A polymer electrolytemembrane made according to the method of claim
 7. 16. A polymerelectrolyte membrane made according to the method of claim
 8. 17. Apolymer electrolyte membrane made according to the method of claim 9.18. A polymer electrolyte membrane made according to the method of claim10.
 19. A polymer electrolyte membrane made according to the method ofclaim
 11. 20. A polymer electrolyte membrane made according to themethod of claim
 12. 21. A polymer membrane comprising a highlyfluorinated polymer comprising: a perfluorinated backbone, first pendentgroups which comprise groups according to the formula —SO₂X, where X isF, Cl, Br, OH, or —O⁻M⁺, where M⁺ is a monovalent cation, and crosslinkscomprising trivalent groups according to the formula:


22. The polymer membrane according to claim 1 wherein said first pendentgroups are according to the formula: —R¹—SO₂X, where R¹ is a branched orunbranched perfluoroalkyl or perfluoroether group comprising 1-15 carbonatoms and 0-4 oxygen atoms.
 23. The polymer membrane according to claim1 wherein said first pendent groups are according to the formula:—O—CF₂—CF₂—CF₂—CF₂—SO₂X.
 24. The polymer membrane according to claim 1wherein said first pendent groups are according to the formula:—O—CF₂—CF(CF₃)—O—CF₂—CF₂—SO₂X.
 25. A polymer comprising a highlyfluorinated polymer comprising: a perfluorinated backbone, first pendentgroups which comprise groups according to the formula —SO₂X, where X isF, Cl, Br, OH, or —O⁻M⁺, where M⁺ is a monovalent cation, and crosslinkscomprising trivalent groups according to the formula:


26. The polymer according to claim 1 wherein said first pendent groupsare according to the formula: —R₁—SO₂X, where R¹ is a branched orunbranched perfluoroalkyl or perfluoroether group comprising 1-15 carbonatoms and 0-4 oxygen atoms.
 27. The polymer according to claim 1 whereinsaid first pendent groups are according to the formula:—O—CF₂—CF₂—CF₂—CF₂—SO₂X.
 28. The polymer according to claim 1 whereinsaid first pendent groups are according to the formula:—O—CF₂—CF(CF₃)—O—CF₂—CF₂—SO₂X.